13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Scaling invariance of fatigue crack growth in gigacycle regime Vladimir Oborin1,*, Mikhail Bannikov1, Oleg Naimark1, Thierry Palin-Luc2 1 Institute of Continuous Media Mechanics of Ural branch of RAS, 614013, Perm, Russia 2 I2M-DuMAS CNRS, Université de Bordeaux, Arts et Métiers-ParisTech, France *oborin@icmm.ru Abstract The role of the collective behavior of defect ensembles at the crack tip and the laws of fatigue crack propagation in R4 high-strength steel have been studied under conditions of symmetric tension–compression gigacycle loading at 20 kHz. At every stage of the fatigue crack growth, replicas from the sample side surface were taken and studied by the method of three-dimensional relief profilometry (using interferometer profilometer NewView 5010) so as to study the scaling-invariant laws of defect-related structure evolution. Keywords gigacycle fatigue, scaling, crack, morphology. 1. Introduction The task of assessing the working resource of important structures, in particular, those for aircraft engines, poses qualitatively new basic problem related to evaluation of the reliability of materials under conditions of cyclic loading in excess of 108–1010 cycles, which refer to the field of so-called gigacycle fatigue. This interest is related to the fact that the resource of loading for many important parts operating under conditions of cyclic loading exceeds the so-called multicycle range. The behavior of materials in the range of gigacycle fatigue reveals some qualitative changes in the laws governing both the nucleation of cracks (in the bulk of a sample) and their propagation, which are related to changes in the mechanisms of fatigue crack nucleation and propagation. In the range of gigacycle loading, the fatigue curve exhibits discontinuities and the behavior shows evidence of a significant increase in the role of environment, so that the problem acquires an interdisciplinary character. The stages of material fracture in the range of gigacycle loading are classified based on the structural signs of damage related to a broad spectrum of spatial scales, including persistent slip bands (PSBs), fatigue striations, microcracks (formed as a result of PSB crossing), and grain-boundary defects. The main damage refers to the defect scales within 0.1 μm–1 mm, which are significantly smaller than those detected by the standard methods of nondestructive testing used for the conventional monitoring of reliability, in particular, during the exploitation of buildings. An effective method for investigating the role of initial structural heterogeneity, monitoring the accumulation of defects on various scales (dislocation ensembles, micropores, microcracks), and determining critical conditions for the transition from dispersed to macroscopic fracture is offered by the quantitative fractography. This technique reveals the characteristic stages of fracture (crack nucleation and propagation), thus providing a base for evaluating the temporal resource of materials and structures under conditions of gigacycle loading. The approach to characterization of the fracture surface morphology in terms of spatiotemporal invariants was originally proposed by Mandelbrot [1]. This method is based on an analysis of the relief of a fracture surface, which exhibits the property of self-affinity as manifested by the invariant characteristics of the surface relief (roughness) over a broad spectrum of spatial scales. On the other hand, these characteristics reflect a correlated behavior of defects on various scaling levels. The universal character of kinetic laws establishing a relationship between the growth rate dl/dN of a fatigue cracks and a change in the stress intensity coefficient ΔK has been extensively studied both
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